Hi
YK •,
Welcome to Microsoft Q&A forum.
As I understand, you are unable to add database to failover group and shows it is busy with other operation.
I would recommend you to wait for sometime and retry.
Even after waiting if the issue exists, I would recommend you to please file a support ticket for deeper investigation and in case if you don't have a support plan, do let us know here so that we can check on other options to unblock you.
Thanks